A mountain as grueling as Everest is the last place you’d expect to encounter traffic. But with a single icy path to the mountain’s narrow summit and few days of clear weather to climb, there can be periodic rushes to make it to the top of the world. And under such treacherous conditions, not moving quickly enough can be fatal.

Four climbers were killed Saturday as they mounted the world’s highest summit, 29,035 feet (8,850 meters) above sea level. Favorable weather conditions led to a major scramble as 208 climbers attempted the trek to the top from the last high-altitude camp at 26,246 feet (8,000 meters) – more, it turned out, than the mountain could safey accommodate.

“Climbers had a longer wait for their chance to go up the trail and spent too much time at higher altitude,” Nepali mountaineering official Gyanendra Shrestha told the Associated Press. “Many of them are believed to be carrying a limited amount of oxygen, not anticipating the extra time spent.” The four climbers, who hailed from Germany, Canada, South Korea and China, are believed to have succumbed to altitude sickness, lack of oxygen and exhaustion. A Nepalese Sherpa guide was declared missing along with the four but managed to descend the mountain to safety.

Office Seven Summits / File / AP

All climbers must have permits to climb within a specific date range, but the unpredictable weather conditions mean that they don’t have an exact date to reach the top. Climbers can spend days at the camps scattered up the mountain just waiting for weather to improve. The climbing season begins in late March and runs through June, but this year, the first clear days for climbing were on May 19 and 20, the Associated Press reported. “The window of good weather is so short,” said Apa, a Nepalese Sherpa guide who uses only one name.

Despite the tragedy, one woman had reason to celebrate. Tamae Watanabe, 73, set a record as the oldest woman to reach the summit of Everest. It was a record she claimed from, well, herself: Watanabe first entered the Guinness Book of World Records a decade ago at the age of 63; she held the title until she one-upped herself Saturday. (She’s not the oldest person ever to make the climb, though: that record is held by Nepalese climber Min Bahadur Sherchan, who reached the peak in 2008 at the age of 76.) The Japanese citizen was said to be in “good condition” as she and her four-person team made it back to base.

Edmund Hillary and Tenzing Norgay first reached the top of Mount Everest in 1953. Since then, more than 3,000 have followed suit – and 225 lost their lives in the attempt. But last weekend’s deadly outcome isn’t expected to scare away too many climbers. Officials predict another 200 climbers are expected to attempt the summit this weekend.